Asynchronous means "not synchronous". Synchronous means "agreed timing for the sending of ones and zeroes (bits)"--that is, the transmit and receive sides of the communications circuit have bothered to coordinate (synchronize) their signal and have agreed just what a digital bit encoded into the signal looks like. All communications paths have carrier signals, the signals have a frequency, and encoding bits into the signal involves spacing them out at regular intervals, and carving out just how long it takes to transmit a bit (well, it gets a bit more complicated than that, but this is a 'basic' tutorial after all).

So, which is better, Asynchronous or Synchronous?

It depends.

Let's look at Synchronous and Asynchronous in a little more detail.

Asynchronous (Asynchronous I/O or Asynchronous Transmission)

Asynchronous communication utilizes a transmitter, a receiver and a wire without coordination about the timing of individual bits. There is no coordination between the two end points on just how long the transmiter leaves the signal at a certain level to represent a single digital bit. Each device uses a clock to measure out the 'length' of a bit. The transmitting device simply transmits. The receiving device has to look at the incoming signal and figure out what it is receiving and coordinate and retime its clock to match the incoming signal.

Sending data encoded into your signal requires that the sender and receiver are both using the same encoding/decoding method, and know where to look in the signal to find data. Asynchronous systems do not send separate information to indicate the encoding or clocking information. The receiver must decide the clocking of the signal on it's own. This means that the receiver must decide where to look in the signal stream to find ones and zeroes, and decide for itself where each individual bit stops and starts. This information is not in the data in the signal sent from transmitting unit.

When the receiver of a signal carrying information has to derive how that signal is organized without consulting the transmitting device, it is called asynchronous communication. In short, the two ends do not always negotiate or work out the connection parameters before communicating. Asynchronous communication is more efficient when there is low loss and low error rates over the transmission medium because data is not retransmitted and no time is spent setting negotiating the connection parameters at the beginning of transmission. Asynchronous systems just transmit and let the far end station figure it out. Asynchronous is sometimes called "best effort" transmission because one side simply transmits, and the other does it's best to receive and any lost data is recovered by a higher level protocol.


  • Asynchronous communication is used on RS-232 based serial devices such as on an IBM-compatible computer's communication ports (COM 1, 2, 3 & 4) for the following functions:
    • PS2 ports on your computer use asynchronous serial communication with the keyboard and mouse
    • The serial port is used to communicate with an external devices such as modems.
  • Asynchronous Transfer Mode (ATM) also uses asynchronous communication.

Think of asynchronous as a faster means of connecting, but less reliable when transmitting.

Synchronous (Synchronous I/O or Synchronous Transmission)

Synchronous systems negotiate the communication parameters at the data link layer before communication begins. Basic synchronous systems will synchronize the signal clocks on both sides before transmission begins, reset their numeric counters and take other steps. More advanced systems may negotiate things like error correction and compression.

It is possible to have both sides try to synchronize the connection at the same time. Usually, there is a process to decide which end should be in control. Both sides in synchronous communication can go through a lengthy negotiation cycle where they exchange communications parameters and status information. With a lengthy connection establishment process, a synchronous system using an unreliable physical connection will spend a great deal of time in negotiating, but not in actual data transfer. Once a connection is established, the transmitter sends out a signal, and the receiver sends back data regarding that transmission, and what it received. This connection negotiation process takes longer on low error-rate lines, but is highly efficient in systems where the transmission medium itself (an electric wire, radio signal or laser beam) is not particularly reliable.

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